1. Field of the Invention
The invention relates generally to a device for operating a high pressure discharge lamp. The invention relates more specifically to an ultra-high pressure AC discharge lamp in which an arc tube is filled with greater than or equal to 0.15 mg/mm3 mercury, in which the mercury vapor pressure during operation is greater than or equal to 110 atm, and that can be used as a projection light source for a projection type projection device or the like.
2. Description of the Related Art
In projection-type projector devices there is a significant demand to be able to illuminate images onto a rectangular screen in a uniform manner and with adequate color rendering. The light source is a metal halide lamp filled with mercury and a metal halide. As the projection devices have developed, the size of metal halide lamps has decreased and more light sources have been produced employing extremely small distances between the electrodes.
Recently, instead of metal halide lamps, high-pressure discharge lamps with an extremely high mercury vapor pressure, for example with greater than or equal to 200 bar (197 atm), have been used. By using high-pressure discharge lamps, the broadening of the arc is suppressed by increased mercury vapor pressure, and the arc is compressed and a great increase of light intensity results.
Recently, there has been a focus on smaller and smaller projector devices. In the discharge lamp for the above-described projector device, on the one hand, there has been a demand for high light intensity and the ability to maintain illuminance. On the other hand, due to the reduction in size of the projector device, there is also a demand for smaller discharge lamps. Therefore, smaller devices and smaller power sources are being used. Thus, a reduction in the voltage during starting (i.e., a property to facilitate starting) is expected.
For the above-described lamp, for example, an ultra-high pressure discharge lamp is used. Located in a silica glass arc-tube is a pair of electrodes a distance of less than or equal to 2 mm apart. The arc-tube is filled with greater than or equal to 0.15 mg/mm3 mercury, rare gas and halogen in the range from 1×10−6 μmole/mm3 to 1×10−2 μmole/mm3 (for example, see U.S. Pat. No. 5,109,181 (corresponding to JP-A-2-148561) and U.S. Pat. No. 5,497,049 (corresponding to Japanese patent specification 2980822)). One such discharge lamp and the operating device for it are disclosed, for example, in U.S. Pat. No. 6,545,430 (corresponding to JP-A-2001-312997).
In the high pressure discharge lamp disclosed in U.S. Pat. No. 6,545,430 B2, at a mercury vapor pressure within the tube of 15 MPa to 35 MPa in steady-state operation, the arc tube is filled with a halogen material in the range from 1×10−6 μmol/mm3 to 1×10−2 μmol/mm3. Placing a pair of electrodes within the arc tube and placing a projection in the vicinity of the middle of the electrode tip area suppresses the arc jump phenomenon. An AC voltage is applied by an operating device which consists of a DC/DC converter, a DC/AC inverter and a high voltage generation device between the pair of electrodes.
In such an ultra-high pressure discharge lamp, the phenomenon that occurs on the tips of the opposed tungsten electrodes in the arc tube is that, during operation, projections are formed and grow. These projections arise and grow dramatically especially if AC operation is carried out with a distance between the electrodes of less than or equal to 1.5 mm, an amount of mercury of greater than or equal to 0.15 mg/mm3 and an amount of halogen (e.g., bromine or the like) of 10−6 μmol/mm3 to 10−2 μmol/mm3. The phenomenon in which the projections are formed on the electrode tips cannot always be unambiguously explained, but the following can be assumed.
In one such discharge lamp, the arc tube is filled with a halogen gas. The main objective is to prevent devitrification of the arc tube. The halogen gas also yields the so-called halogen cycle. The tungsten, which during lamp operation is vaporized from the area with a high temperature in the vicinity of the electrode tip, reacts with the halogen and the remaining oxygen which is present within the arc tube, and a tungsten compound is formed such as WBr, WBr2, WO, WO2, WO2Br, WO2Br2 or the like, if, for example, the halogen is Br. These compounds decompose in the area with a high temperature in the gaseous phase in the vicinity of the electrode tip, and become tungsten atoms or cations. The tungsten atoms are transported by thermal diffusion (diffusion of the tungsten atoms from the high temperature region in the gaseous phase, (i.e., from the arc) to the low temperature region, (i.e., the vicinity of the electrode tip)) and in the arc, become cations and, during operation of the cathode, are pulled by the electrical field in the direction to the cathode (drift). In this way, the density of the tungsten vapor in the gaseous phase in the vicinity of the electrode tip is increased and is precipitated on the electrode tip, thereby forming projections.
These projections have the effect that they can prevent the arc jump. If, in the course of continued operation of the lamp, the projections grow, the disadvantage arises that the distance between the electrodes is reduced, the position of the arc radiance spot is changed and that the light intensity is reduced.
In the above-described U.S. Pat. No. 6,545,430 B2 it is shown that by the formation of the projection, the lamp voltage fluctuates (decreases). Furthermore, it is disclosed that in the case of a change in the lamp voltage (i.e., the distance between the electrodes) by the formation of the projection by controlling the amount of current flowing between the two electrodes and by switching the first frequency of the operation frequency to a second frequency, the fluctuation of the lamp voltage by the formation of the projection is corrected.
For example, with respect to the amount of current flowing between the two above-described electrodes, the following is shown:
In the case in which the lamp voltage (i.e., distance between the electrodes) becomes smaller than the normal value, the length of the projection is reduced by increasing the discharge arc current which flows between the two electrodes, by which the lamp voltage increases (i.e., rises). In the case in which the lamp voltage (i.e., distance between the electrodes) becomes greater than the normal value, the length of the projection is increased by the reduction of the discharge arc current.
Based on these ideas, in the operating device described in U.S. Pat. No. 6,545,430 B2, a higher discharge arc current is allowed to flow if the determined lamp voltage is less than the reference voltage. Furthermore, the above-described DC/DC converter is controlled with feedback such that the discharge arc current is reduced when the lamp voltage is higher than the reference voltage. Thus, the fluctuation of the lamp voltage is suppressed.
It can be envisioned that the control of the change of the distance between the electrodes by the discharge arc current described in U.S. Pat. No. 6,545,430 B2 is effective in certain cases. It was, however, found that the growth of the projections cannot be advantageously controlled.
In U.S. Pat. No. 6,545,430 B2 a higher discharge arc current is allowed to flow in the case in which the determined value of the lamp voltage is lower than the reference voltage. Furthermore, the discharge arc current is reduced when the value of the lamp voltage is higher than the reference voltage. As a result, it was however found that the growth of projections cannot always be advantageously controlled by this type of control. U.S. Pat. No. 6,545,430 especially discloses a process for two-stage alteration of the discharge current. Since in this control the lamp voltage changes rapidly, as can be imagined, stable maintenance of the lamp voltage and of the distance between the electrodes becomes difficult.